1. Field of the Invention
The present invention relates to a nickel-metal hydride secondary battery. More particularly, the present invention is concerned with a nickel-metal hydride secondary battery which is advantageous not only in that it exhibits high charging efficiency in a high temperature environment and high capacity maintaining rate due to the excellent self-discharge characteristics thereof in a high temperature storage, high utilization of the active material for positive electrode, and further it can exhibit a large current discharge even at the initial stage of the discharge, but also in that it has excellent low-temperature charging characteristics.
2. Prior Art
In recent years, various electronic appliances, such as pocketable telephones and hand-held personal computers, have been developed in respect of being cordless, multifunctionality and compactness as well as weight reduction. In accordance with these developments, the demand for high capacity of a second battery which is a power source of the electronic appliance has greatly increased.
Conventionally, as the power sources of these electronic appliances, nickel-cadmium secondary batteries have been mainly used. However, in accordance with the above-mentioned great demand for high capacity, recently, a nickel-metal hydride secondary battery is beginning to be widely used because the nickel-metal hydride secondary battery is interchangeable with the voltage of the nickel-cadmium secondary battery and has a higher capacity than that of the nickel-cadmium secondary battery.
This nickel-metal hydride secondary battery generally comprises an electrode group comprising: a positive electrode comprising a current collector having carried thereon a mixture paste containing, as a main component, a nickel hydroxide powder which is an active material and a binder such as carboxymethyl cellulose; a negative electrode comprising a current collector having carried thereon a paste containing, as a main component, a powder of a hydrogen storage alloy and a binder such as carboxymethyl cellulose or polytetrafluoroethylene; and a separator comprising, for example, a polyamide fiber nonwoven fabric, having electrical insulating properties and a liquid retaining property, which separator is disposed between the positive electrode and the negative electrode, and has a structure such that this electrode group is accommodated in a battery casing which also serves as a negative electrode terminal, together with an alkali electrolyte liquid which is generally comprised mainly of an aqueous potassium hydroxide solution, and then, the battery casing is sealed up so that the electrode group and the alkali electrolyte liquid are sealed in the casing.
Then, an initial charge is conducted with respect to the assembled battery, so that nickel hydroxide as an active material for positive electrode is treated for activation. That is, this is a treatment such that nickel hydroxide which itself has no conductivity is converted into xcex2-nickel oxyhydroxide being of trivalent and having a conductivity by subjecting to an initial charge for oxidation, making it possible to exhibit a function as an active material.
By the way, this nickel-metal hydride secondary battery is operated utilizing the characteristics of the hydrogen storage alloy such that it absorbs and desorbs hydrogen electrochemically and reversibly.
However, when this nickel-metal hydride secondary battery in a charging state is allowed to stand or stored in a high temperature environment, generally, the equilibrium pressure of the hydrogen storage alloy in the negative electrode increases, and thus, the amount of hydrogen which can be occluded in the negative electrode is reduced. Therefore, the hydrogen which cannot be stored any more in the negative electrode is emitted to the inside of the battery, so that the hydrogen partial pressure in the battery is increased. Then, this hydrogen passes through the separator to the positive electrode, and at the positive electrode, promotes the reduction of the nickel oxyhydroxide present in the positive electrode in a state of xcex2-nickel oxyhydroxide which is an oxide product from nickel hydroxide by the above-mentioned initial charge. That is, the self-discharge of the nickel oxyhydroxide is promoted, and as a result, lowering of the discharge capacity occurs.
For solving such a problem, there has been proposed a method in which a vinyl monomer having a carboxyl group is subjected to graft polymerization on the surface of the separator comprising a fiber of a polyolefin resin (see Japanese Unexamined Patent Publication No. Hei 10-69898). By this method, the separator is provided with high hydrophilicity. Therefore, on the surface containing the spaces between the fiber constituting the separator, a film of an alkali electrolyte liquid is formed, and thus, this film prevents a diffusion of hydrogen into the positive electrode even when the hydrogen partial pressure within the battery is increased. As a result, the above-mentioned self-discharge of the positive electrode is suppressed.
However, recently, the use of the nickel-metal hydride secondary battery has been expanded in the application fields, such as an electric power tool, an electric vehicle and an electric power-assist bicycle, as a power source which requires a large current discharge. In view of the strong demand for the improvement of the self-discharge characteristics in the above application field, the method proposed in the above-mentioned prior art document has a problem in that a satisfactory improvement of the self-discharge characteristics cannot be achieved.
On the other hand, from the viewpoint of the charging problem of the nickel-metal hydride secondary battery, when the environment at the charging is a normal temperatures environment, the overpotential at the charging reaction of nickel hydroxide is larger than that required for the oxygen generation reaction from the alkali electrolyte liquid. Therefore, the charging reaction of nickel hydroxide first proceeds, and after almost completion of the charging reaction, the reaction is transferred to the oxygen generation reaction. Accordingly, in a normal temperatures environment, it is possible to advance the charging of the positive electrode surely and satisfactorily.
However, when the charging is conducted in a high temperature environment, the overpotential at the oxygen generation reaction is lowered. Therefore, the difference between the overpotential at the oxygen generation reaction and that at the charging reaction of nickel hydroxide becomes small. For this reason, from the relatively early stage of the charging, the competitive relationship between the charging reaction and the oxygen generation reaction occurs, causing a disadvantage that the charging reaction of nickel hydroxide does not satisfactorily proceed. That is, in a high temperature environment, problems arise in that the charging efficiency is lowered, and as a result, the discharge capacity of the battery is lowered.
For solving such problems, there have been proposed a method using, as an active material, nickel hydroxide having a cobalt component coprecipitated therein during the synthesis of the nickel hydroxide for lowering the equilibrium potential of the positive electrode (see Japanese Unexamined Patent Publication No. Sho 50-132441), and a method using nickel hydroxide having cadmium or the like coprecipitated therein for increasing the oxygen overpotential of the positive electrode (see Japanese Unexamined Patent Publication No. Sho 62-108458).
However, the positive electrodes produced by these methods do not exhibit a satisfactory level of charging efficiency in a high temperature environment.
In addition, in Japanese Unexamined Patent Publication No. Hei 5-28992, a positive electrode produced by adding to nickel hydroxide a compound of Y, In, Sb, Ba, Ca, Be or the like for increasing the oxygen overpotential is disclosed. However, in this positive electrode, due to the addition of the above-mentioned compound which does not participate in the charging and discharging reactions, the relative content of the nickel hydroxide functioning as an active material in the positive electrode becomes small. As a result, a problem of lowering of the discharge capacity arises.
Further, Japanese Unexamined Patent Publication No. Sho 47-20635 discloses a method using an alkali electrolyte liquid containing a tungstic acid ion for improving the charging efficiency in a high temperature environment.
However, in this method, for obtaining a satisfactory charging efficiency, it is necessary to use an alkali electrolyte liquid containing the tungstic acid ion in a higher content. In such a case, the viscosity of the alkali electrolyte liquid increases and the ion mobility is lowered, and as a result, the large current discharge characteristics disadvantageously become poor.
Further, Japanese Unexamined Patent Publication No. HEI 8-88020, Japanese Unexamined Patent Publication No. Hei 8-190931, Japanese Unexamined Patent Publication No. Hei 8-222213, Japanese Unexamined Patent Publication No. Hei 10-172558 and Japanese Unexamined Patent Publication No. Hei 10-24163 disclose nickel-metal hydride secondary batteries containing a tungstic acid ion or tungsten compound.
However, in those batteries, initial utilization of the active material and self-discharge characteristics in a high temperature environment are not satisfactory.
Further, in the charging of the nickel-metal hydride secondary battery, there involves the following disadvantages: since a large difference in current collection efficiency between the surface portion and the central portion of the nickel hydroxide as an active material is caused, when it is presumed that a full charging in a depth of 100% or more is conducted, the surface portion of the nickel hydroxide powder becomes in an overcharge state unavoidably. Therefore, the surface portion is excessively oxidized, and thus, xcex3-nickel oxyhydroxide being of trivalent or more is formed there.
However, this xcex3-nickel oxyhydroxide is inert, and has a low bulk density, as compared to trivalent xcex2-nickel oxyhydroxide. Therefore, it is said that the xcex3-nickel oxyhydroxide has a cause for the lowering of the charging efficiency and the swelling of the positive electrode, as well as a cause for the lowering of the complete discharge capacity (memory effect) after the repetition of the shallow charging and discharging.
In order to suppress the formation of the above-mentioned xcex3-nickel oxyhydroxide, there has been proposed, for example, a method in which the surface of a nickel hydroxide powder is coated with an amorphous layer of a material comprised of nickel hydroxide and manganese hydroxide (see Japanese Unexamined Patent Publication No. Hei 10-106559). However, it is considered that this method poses a problem in that the manganese component which can be easily dissolved in the alkali electrolyte liquid in the battery penetrates into the inner portion of the nickel hydroxide powder, and the formation of the xcex3-nickel oxyhydroxide is rather promoted.
By the way, either in the above-mentioned initial charge or in the charge-discharge cycle process at an actual use, when the conductivity between the active materials for positive electrode and that between the active material for positive electrode and the current collector are high, the utilization of the active material for positive electrode becomes high, so that the reaction for battery smoothly proceeds. Therefore, from the viewpoint of realizing the theoretical capacity of the positive electrode, this is an important task.
For realizing such a task, measures have conventionally been taken as follows.
There is a method in which first, in the preparation of a paste for the positive electrode, a powder of a metallic cobalt or a powder of a cobalt compound, such as cobalt hydroxide, tricobalt tetroxide, dicobalt trioxide, cobalt monoxide or the like, or the mixture thereof is added in a predetermined amount as a conductive material, thereby producing a powder having a nickel hydroxide power mixed therein in a predetermined ratio, and this powder is used as an active material.
In the nickel-metal hydride secondary battery which has incorporated therein the positive electrode having the above-produced active material powder carried therein, any metallic cobalt or cobalt compound contained in the above powder is once dissolved in the alkali electrolyte liquid as complex ions and diffused between the nickel hydroxide powder, so that the complex ions are distributed on the surface of the powder. Then, in the initial charge for battery, these complex ions are oxidized prior to the oxidation of nickel hydroxide and converted into a higher order oxide of cobalt having a conductivity, and the oxide deposits between the nickel hydroxide powder as an active material and between the active material layer and the current collector, so that a conductive matrix is formed. As a result, the conductivity between the active materials and that between the active material and the current collector are enhanced, and hence, the utilization of the active material improves.
Therefore, in order to improve the utilization of the active material, it is considered effective to increase the content of metallic cobalt or a cobalt compound in the above-mentioned active material powder, and thereby increase the amount of the above-mentioned conductive matrix formed.
However, when such a method is employed, there are disadvantages not only in that the production cost for the positive electrode becomes high, but also in that as a whole of the battery, the relative content of the nickel hydroxide functioning as an active material for positive electrode becomes small, and thus, a high capacity battery cannot be obtained.
In view of this, it is preferred that an active material is in a state such that it can satisfactorily exhibit the effects thereof even though the content of the metallic cobalt or cobalt compound is minimized.
By the way, with respect to the improvement of the utilization of the active material, for example, Japanese Unexamined Patent Publication No. Hei 3-78965 discloses a method in which to an aqueous alkaline solution having a pH of 11 to 13 is added a powder comprised mainly of nickel hydroxide, and, for example, an aqueous solution of cobalt sulfate is added thereto, so that the surface of the above powder is coated with cobalt hydroxide (cobalt compound).
According to this method, it becomes possible to coat the surface of the nickel hydroxide powder with a small amount of a cobalt compound. However, on the other hand, there is a problem in that the amount of the above-mentioned conductive matrix formed is decreased, and the conductivity of the whole of the positive electrode does not become satisfactorily large.
Further, Japanese Unexamined Patent Publication No. Hei 9-213326 discloses a method in which a powder comprised mainly of nickel hydroxide is subjected to alkali heat treatment by performing a heat treatment in the coexistence of oxygen simultaneously with adding thereto both of an aqueous alkaline solution and a cobalt-containing aqueous solution, to thereby form, on the surface of the above powder, a layer of a higher order oxide of cobalt containing an alkali metal cation, such as Na+.
It is disclosed that, according to this method, the obtained active material powder has an excellent conductivity and the utilization as an active material improves; however, as a matter of fact, there is a problem in that, at the early step of the above-mentioned alkali heat treatment in the coexistence of oxygen, before the surface of the powder comprised mainly of nickel hydroxide is coated with the cobalt compound, the cobalt compound disadvantageously undergoes oxidization, so that the contact between the cobalt compound and nickel hydroxide is decreased.
In view of above, the present inventors have employed a technique using an irradiation of a microwave from a magnetron as a heat treatment method for the formation of a conductive layer comprised of a higher order oxide of cobalt containing an alkali metal cation on the surface of the powder comprised mainly of nickel hydroxide, and have already filed this technique as Japanese Patent Application No. Hei 10-63142. By this method, a higher order oxide of cobalt having a conductivity can be efficiently formed while suppressing the air oxidization of the cobalt compound during the alkali heat treatment. Therefore, the utilization of the obtained powder as an active material becomes high.
However, as a result of the subsequent studies, it has been found that the nickel-metal hydride secondary battery having incorporated therein a positive electrode which contains, as an active material, the powder produced by the above-mentioned method poses a problem in that, in accordance with the progress of the charge-discharge cycle, the alkali metal cation contained in the conductive matrix comprised of the above-mentioned higher order oxide of cobalt is dissolved in the alkali electrolyte liquid, and as a result, the conductivity of the conductive matrix is lowered, leading to lowering of the utilization of the active material.
It is an object of the present invention to solve the above-mentioned problems accompanying the conventional nickel-metal hydride secondary battery, and provide a nickel-metal hydride secondary battery which is advantageous not only in that it exhibits high utilization of the active material and excellent self-discharge characteristics in a high temperature environment, high charging efficiency in a high temperature environment, and further it can exhibit a large current discharge even at the initial stage of the discharge, but also in that it has excellent low-temperature charging characteristics.
For attaining the above objects, in the present invention, there is provided a nickel-metal hydride secondary battery (hereinafter, referred to as xe2x80x9cbattery Ixe2x80x9d) comprising:
an electrode group comprising a positive electrode having carried thereon an active material powder containing a nickel hydroxide as a main component, a negative electrode having carried thereon a hydrogen storage alloy powder, and a separator being disposed between the positive electrode and the negative electrode, the electrode group being sealed in a battery casing, together with an alkali electrolyte liquid,
wherein, in the battery, a W element and an Na element are present simultaneously.
Specifically, there is provided a nickel-metal hydride secondary battery wherein the W element and the Na element are respectively present as a tungstic acid ion and a sodium ion, and the relationship represented by the following formulae (1) and (2) is satisfied:
0.03xe2x89xa6xxe2x89xa64xe2x80x83xe2x80x83(1)
1.5xe2x89xa6yxe2x89xa6xe2x88x9214x+70xe2x80x83xe2x80x83(2)
wherein x represents a hundredfold value of the atomic ratio of the W element to the total alkali metal element present in the battery, and y represents a hundredfold value of the atomic ratio of the Na element to the total alkali metal element present in the battery.
Further, in the present invention, there is provided a nickel-metal hydride secondary battery (hereinafter, referred to as xe2x80x9cbattery IIxe2x80x9d) comprising:
an electrode group comprising a positive electrode having carried thereon an active material powder containing a nickel hydroxide as a main component, a negative electrode having carried thereon a hydrogen storage alloy powder, and a separator being disposed between the positive electrode and the negative electrode, the electrode group being sealed in a battery casing, together with an alkali electrolyte liquid,
wherein at least W element is present in the battery, and wherein a conductive coating layer comprised mainly of an Na-containing cobalt compound is formed on the surface of the active material powder, and a nickel-metal hydride secondary battery (hereinafter, referred to as xe2x80x9cbattery IIIxe2x80x9d) comprising:
an electrode group comprising a positive electrode having carried thereon an active material powder containing a nickel hydroxide as a main component, a negative electrode having carried thereon a hydrogen storage alloy powder, and a separator being disposed between the positive electrode and the negative electrode, the electrode group being sealed in a battery casing, together with an alkali electrolyte liquid,
wherein at least W element is present in the battery, and wherein the separator comprises a sheet-form member having an acid group introduced into the surface thereof.